This invention relates to a sensor element for detecting trace amounts of gaseous reducing substances such as alcohols, aldehydes, hydrocarbons, carboxylic acids, amines, and carbon monoxide contained in the atmosphere, exhaust gases, the breath, etc., or for determination of oxygen concentration and a method for detecting the gaseous substances.
For detecting trace amounts of reducing gases contained in these atmospheres, there have been conventionally known such methods as gas chromatography and a method of detection by use of a semi-conductor element. As for the gas chromatography, it cannot be said to be a convenient and inexpensive detecting method because it requires a large-scale apparatus and a certain degree of skill for analytical procedures. Among detecting methods which make use of a semiconductor element as the sensor, there has been known, for example, a method which utilizes the change in specific resistance of a shaped piece comprising n-type stannic oxide as the main constituent, resistance change of which takes place by adsorption of ethanol vapor. This method, however, has such disadvantages that the ethanol vapor once adsorbed on the sensor will not be desorbed unless the sensor is heat-treated at a temperature of 350.degree. C. or higher, and, in addition, the sensor has an extremely large temperature coefficient of resistance, viz. about 5 to 10 %/.degree. C.
On the other hand, for detecting oxygen concentration in the atmosphere, there is known a method in which a galvanic cell is employed. This method, however, has such disadvantages that the response is slow, the life of the element is only about 6 months counting from preparation of the element, and the solution contained in the sensing element will raise a problem of maintenance.
There is known also another method for measuring oxygen concentration by the solid-state oxygen concentration cell method which employs an oxygen ion conductive solid electrolyte such as, for example, (Zr, Ca)O.sub. 2.sub.-y. According to this method, the partial pressure of oxygen in a sample gas is measured by the oxygen concentration cell method using gaseous oxygen, in which the oxygen partial pressure Po.sub.2 is 1.0 atm, or air, in which the oxygen partial pressure is 0.21 atm, as the standard gas for reference. This method, however, has such disadvantages that the sensor will not operate with stability unless the temperature is above about 800.degree. C., and the output voltage is low in case oxygen concentration of the sample gas approximates that of the standard gas.